1. Field of the Invention
This invention relates to a light emitting device and a method of fabricating the light emitting device.
2. Description of the Related Art
[Patent Document 1] Japanese Laid-Open Patent Publication No. H2-260671
[Patent Document 2] Japanese Laid-Open Patent Publication No. H9-27498
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2003-142731
[Non-Patent Document 1] “Development of Highly Reliable Sn—Ag Lead-Free Solder Alloy”, The R&D Review, Toyota Central Research Labs., Inc., Vol. 35, No. 2 (2000), p. 39”
Light emitting devices composed of compound semiconductors have long been configured, while assuming the main surface thereof positioned on the light extraction side as a first main surface, and the main surface thereof opposite thereto as a second main surface, so that electrodes on the second main surface side, which corresponds to the back surface of a device chip, are bonded to a metal stage using Ag paste (Patent Document 1), but there has been an ongoing discussion on shifting into mounting process based on solder reflow, similar to a so-called flip chip method, in view of improving process efficiency in chip mounting (Patent Document 2). “Solder” generally referred to in mounting process of electronic devices means Pb—Sn-base solder, and in particular, an eutectic solder composed of a low-melting-point Pb-38%-by-mass-Sn alloy (mp. 183° C.) has widely been used. However, recent increasing demands are directed to use of low-Pb solder containing no Pb (or containing only a smaller content of Pb than in the eutectic solder), in place of conventional Sn—Pb eutectic solder, in view of environmental pollution.
As the low-Pb solder substitutive to the conventional Pb-base solder, solders mainly containing Sn have been considered (Non-Patent Document 1), but they have not so extensively been considered on applications thereof to mounting of light emitting device chips, because of concern of oxidation of the solders mainly composed of Sn. More specifically, Au—Sn-base solder has been proposed as a solder suitable for mounting light emitting device chips, by virtue of its excellence also in corrosion resistance (Patent Document 3).
In the structure of mounting described in Patent Document 3, a Au-base alloyed contact layer (alloyed layer) for ensuring ohmic contact is formed on the second main surface side of a light emitting device chip, a Mo layer is disposed so as to prevent diffusion to or from the alloyed contact layer, a AuGe alloyed layer is further disposed as a metal film having the melting point higher than that of the Au—Sn-base solder, and on the AuGe alloyed layer, there is disposed a structure having a Au—Sn-base solder layer to be reflowed formed thereon. According to the description in Patent Document 3, the Mo layer was conventionally covered with an Au layer, and the Au—Sn-base solder layer was formed on the Au layer, but the Au layer is consumed by the Au—Sn-base solder layer during the reflow process, bringing the Au—Sn-base solder layer into direct contact with the Mo layer, lowering the adhesiveness, and thereby making the Au—Sn-base solder layer more likely to cause delamination from the Mo layer. Therefore, a countermeasure is such as disposing, in place of the Au layer, an AuGe alloyed layer having a higher melting point (that is, not fused during the reflow process) than that of the Au—Sn-base solder, so as to avoid direct contact between the Au—Sn-base solder layer and the Mo layer, and to thereby improve the adhesiveness.
Investigations by the present inventors have, however, revealed that the problem of delamination of the Au—Sn-base solder layer cannot readily be solved, even if the configuration described in Patent Document 3 is adopted. Patent Document 3 exemplifies poor adhesion between the Au—Sn-base solder layer and the Mo layer as a causal factor of the delamination, however in practice, it is supposed that, due to hardness and poor ductility of the Au—Sn-base solder layer per se, and also due to relatively poor ductility of the AuGe alloy as an object to be adhered to, the materials cannot absorb stress induced by impact or thermal hysteresis by their plastic deformation, and result in delamination.
It is therefore an object of this invention to provide, in conjunction with the light emitting device on the premise of mounting by aid of the Au—Sn-base solder layer, a device structure (and a light emitting device module using the same) excellent in reliability of bonding between the Au—Sn-base solder layer and the alloyed contact layer, and less causative, for example, of delamination of the Au—Sn-base solder layer, and a method of fabricating the same.